(a) Field of the Invention
The present invention relates to a ceramic filter used for removing, for instance, non-metallic inclusions which are mixed in or present in molten metals, in particular, a molten metal of aluminum or an aluminum alloy (generically, both of them will herein simply be referred to as "aluminum molten metal") as well as a method for preparing the ceramic filter.
(b) Description of the Prior Art
In general, foreign substances such as non-metallic inclusions are mixed in molten metals, such as the aluminum molten metal. The presence of these foreign substances causes processing problems such as formation of hard spots during casting. For this reason, these foreign substances should desirably be removed from the molten metals prior to the casting to thus improve the quality of the resulting cast articles and to eliminate any trouble possibly observed during post-processing steps. To this end, the molten metal has been pre-treated through filtration and there have been known a variety of devices used for filtering the molten metal, for instance, such a tube filter produced by sintering with an inorganic binder as disclosed in Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") No. Sho 52-22327.
In most of such conventional filters, however, large amounts of SiO.sub.2 and/or CaO are included in the binders used therein and as a result, Si and/or Ca present in the binder may be released into the aluminum molten metal to thus contaminate the molten metal. In addition, the hot bending strengths of these filters (as determined at 800.degree. C.) are low, on the order of about 3 MPa, and this may damage the filters during filtering a molten metal having a high temperature.
Filters whose binder is free of any SiO.sub.2 and which are, accordingly free of any release of Si into the aluminum molten metal are disclosed in J.P. KOKOKU Nos. Hei 5-86459 and Hei 5-86460 and Japanese Un-Examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Hei 2-34732. These filters do not suffer from such a drawback as the contamination of the molten metal due to the release of Si, but they suffer from various other problems. For instance, they are inferior in the ability of passing the aluminum molten metal therethrough, easily cause clogging and they show scattering in the treated amount of the molten metal because of low wettability of the filters by the aluminum molten metal. In addition, the initial pressure head of the aluminum molten metal becomes high and accordingly, the filtering device must sometimes be improved. Moreover, these filters may be damaged during using the same since they have low strength because of insufficient bonding of aggregates through such a binder.
Under such circumstances, there are disclosed filters which make use of a binder containing a desired amount of SiO.sub.2 in, for instance, J.P. KOKAI Nos. Hei 5-138339 and Hei 9-29423, in order to solve these problems such as the release of Si and/or the clogging of these filters. In the filter disclosed in J.P. KOKAI No. Hei 5-138339, however, the binder used comprises a large amount of a crystalline component and this leads to the formation of a highly viscous binder having poor flowability and, in turn, to the localization of the binder on the aggregates surface and the formation of uneven aggregate surface. Accordingly, if the aggregate particles are small, the unevenness of the aggregate surface often results in clogging of pores of the filter and the clogging thus makes the passage of the aluminum molten metal therethrough difficult. Furthermore, the binder comprises 15 to 25% of SiO.sub.2, this in turn requires the sintering at a temperature of higher than 1300.degree. C. to prevent any reduction in the hot strength of the resulting filter. In this case, however, gross crystals are present in the binder and this leads to a decrease in the resistance of the filter to the corrosion by the aluminum molten metal.
The filter disclosed in the foregoing J.P. KOKAI No. Hei 9-29423 may cause contamination of the aluminum molten metal during using the same because of the relatively high SiO.sub.2 content, on the order of 25 to 35% by weight, of the binder used therein. Moreover, the hot bending strength (at 800.degree. C.) of the resulting filter is reduced and this may accordingly result in crushing thereof during filtering a molten metal having a high temperature.
Moreover, J.P. KOKAI No. Hei 4-231425 discloses a filter having high hot bending strength. The binder used in this ceramic filter comprises 2 to 6% by weight of SiO.sub.2, 1 to 10% by weight of MgO, 1 to 8% by weight of TiO.sub.2 and the balance of Al.sub.2 O.sub.3. In other words, the SiO.sub.2 content of the binder is low and accordingly, the filter has low wettability with the aluminum molten metal. For this reason, this makes the passage of the aluminum molten metal therethrough difficult and may lead to insufficiency in the amount of the filtered molten metal and a rise of the initial pressure head of the aluminum molten metal. This may, in turn, sometimes require the improvement of the filtering device. In addition, the binder material has a high viscosity and insufficient flowability and this makes the production of uniform filters difficult and the resulting filter is liable to easily cause clogging. Accordingly, problems of, for instance, unstable throughput of the filter arise.
J.P. KOKOKU No. Sho 47-36347 discloses a filter produced using a binder essentially consisting of 15 to 80% by weight of B.sub.2 O.sub.3, 5 to 50% by weight of CaO, 2 to 60% by weight of Al.sub.2 O.sub.3 and not more than 10% by weight of SiO.sub.2. The CaO present in the binder is not reactive with aluminum, but the CaO per se may easily be mixed in the aluminum molten metal and the CaO mixed in the molten metal would become a cause of, for instance, crack-formation (selvage cracks). Therefore, the binder used is preferably free of any CaO.